Conventionally, aluminum or an aluminum alloy has generally been used as a wiring material for a display device such as a flat panel display. With upsizing and resolution enhancement of the display, however, such aluminum wiring materials became to raise a problem of signal delay due to the properties thereof such as wiring resistance, causing difficulty in displaying a uniform screen.
While copper has an advantage over aluminum in terms of lower resistance, it causes a problem of insufficient adhesion between the substrate (for example, a glass substrate) and the copper when used for gate wiring. In addition, in a case where copper is used for source-drain wiring, there are problems as follows: copper may diffuse into the underlying silicon semiconductor film; copper may be oxidized due to diffusion of oxygen from the oxide semiconductor film; and the like.
In order to solve the above-described problems, multilayer-film wiring has been studied in which a copper layer is provided via a barrier film made of a metal having high adhesion to a substrate (for example, a glass substrate) and also having a barrier property for preventing diffusion into the semiconductor film. As metals having both adhesion and a barrier property, metals such as molybdenum and titanium are known. The multilayer-film wiring employs a two-layer multilayer film in which a layer made of copper and a layer made of a metal having both adhesion and a barrier property or an alloy of such metal are laminated, or a three-layer multilayer film in which a layer made of a metal such as molybdenum or titanium or an alloy thereof is further laminated on said copper layer in order to prevent oxidation of the layer made of copper.
Copper- and molybdenum-containing multilayer-film wiring can be obtained by forming the above-described multilayer film on a substrate (for example, a glass substrate) by a film formation process such as sputtering, and subjecting the resultant to etching using a resist as a mask to form an electrode pattern.
Etching processes include wet etching that uses an etchant and dry etching that uses an etching gas such as plasma. Characteristics such as follows are required for the etchant used for wet etching:                high processing accuracy;        highly stabile and safe components and easy handling;        stable etching performance; and        good wiring configuration should result after etching.        
Generally, as an etchant used in the step of etching copper, an acidic etchant containing hydrogen peroxide and an acid and an acidic etchant containing peroxosulfate and an acid are known. However, if such an etchant containing hydrogen peroxide or peroxosulfuric acid is used, there is a problem of generation of gas and heat due to decomposition of hydrogen peroxide or peroxosulfuric acid. There is also a problem of change in the etching performance due to decomposition of the components.
As peroxide-free etchants for copper, an ammonia alkaline etchant containing a copper (II) ion and ammonia is known. Such ammonia alkaline etchants are also capable of etching a multilayer film containing copper. However, since this etchant has high pH, a large amount of ammonia is volatilized from this etchant and thus the ammonia concentration is decreased, which may result in fluctuation in the etching rate or significant aggravation of the working environment. In addition, there is also a problem of dissolution of the resist if the pH is high.
As an etchant for selectively etching a copper layer in a multilayer film comprising a metal oxide layer containing an oxide of a metal selected from Zn, Sn, Al, In and Ga and the copper layer, an etchant containing a copper (II) ion, an organic acid and an amino group-containing compound and having pH of 5.0-10.5 has been proposed (Patent Document 1). This document, however, does not mention about etching a multilayer film containing copper and molybdenum. Moreover, although this etchant is capable of removing copper, its removal property against molybdenum is low (see Comparative Example 3). Thus, it is not suitable for etching a multilayer film containing copper and molybdenum.
As an etchant for copper or a copper alloy, an etchant containing a copper (II) ion, aliphatic carboxylic acid, a halogen ion and alkanolamine has been proposed (Patent Document 2).
Additionally, as an etchant for copper or a copper alloy, an etchant containing a copper (II) ion, an organic acid ion and maleic acid ion has been proposed (Patent Document 3). This document describes that said etchant can also be applied to etching of a multilayer film including copper and molybdenum.
Meanwhile, recently, in order to meet the requirements of upsizing, high-definition and low power consumption of the display, an oxide semiconductor (IGZO) having a structure in which the semiconductor layer beneath the wiring is composed of indium (In), gallium (Ga) and zinc (Zn) has been studied.
If IGZO is employed for a semiconductor layer, post annealing at a high temperature is required for improving reliability, which leads to oxidization of copper as the wiring material and causes a problem of increasing the wiring resistance. Accordingly, in order to prevent copper oxidization, a multilayer structure using molybdenum as a upper metal cap layer on copper has been studied (for example, a multilayer film of molybdenum/copper/molybdenum), in which case, molybdenum needs to be formed into a thick film in order to prevent copper oxidization by post annealing at a high temperature.
However, when a conventional copper/molybdenum etchant is used to etch the above-mentioned copper- and molybdenum-containing multilayer film in which molybdenum is formed into a thick film, there are a problem that the removal property against molybdenum is insufficient such that the upper molybdenum layer may remain as an eave and a problem that the underlying molybdenum layer may remain due to tailing. Therefore, there has been a need for an etchant that is capable of etching a multilayer containing copper and a thickly-formed molybdenum to have a good etching configuration.